Litter Box Issue - Narrowly Defined Problem (Infrequent / Same Place & Time)

I know there are numerous posts about litter box problems, but I have some bullet points I am hoping might be interesting if anyone has thoughts.

Symptoms:
My female fixed cat is five years old, is in good health, but has had some teeth removed. I have another mail fixed larger Cat who can be aggressive and they are both indoors and have lived together for all their lives.

Symptoms
1. She "Occasionally" urinates on the bed where she sleeps from once a month to once in two months.
2. It is in the morning after waking up.
3. It is a lot of urine.
4. The other cat also sleeps on the bed.
5. She does not have accidents anywhere else
6. She does not have accidents when I am not home.
7. I was experimenting with different kinds of food.
8. I was using scented candles around the time this started which was in May, 2016.
9. February, Something happened to her tail where a large lump formed and virtually disappeared. It is a very small lump. The vet said it seems OK and all nerves seem unaffected.
10. I live in West Los Angeles on a very busy street five floors up, but that has been the case all the cat's lives.
11. She is not timid and when the other cat jumps, she does run away, but not very far. She does not hide, but jumps on things like the trashcan. Both cats eat together and sleep near (but not cuddling) each other.
12. Its as if she just can't hold it.
13. Sometimes she will go out and eat and come back into the room and if I see her jump on the bed and walk around I will take her to her box. To reiterate, she does sleep on the bed near my other cat.
14. I only know of one time she had an accident not on the bed and that was the first day I brought her home five years ago. She had a number of accidents but she was only two months old and the box was upstairs.

One thinks this might be a good set of points different from everyone else's but if not, that gives me more reading and searching opportunities.

Sorry to see there are no replies by now. Let me have a go.

First, what kind of tests did the vet perform to rule out a medical problem? What I'm thinking about is some level of inflammation which is like a "irritable bladder", triggering the urination in some locations and situations (you around) and not others. Did her urine look ok on all fronts? No crystals? No signs of infection?

Next, could you share some more information about the litterbox setup in your home? How many boxes, what type, type of litter and frequency of cleaning. Also, if you can describe the location with as many details as possible. What can she see when using the box? does she have an easy way out? Any changes made to the litterboxes setup since May?

Oh, and as for more resources, we have this guide (you can follow the links in it if you see something which may be applicable in your situation).
[article="32366"]How To Solve Litterbox Problems In Cats The Ultimate Guide  [/article]

Let me know if you need more detail, but it was a Urinalysis that cost around $200.  This was done some time ago (Dec, 2014) and came up negative.  I just took her to same vet today (10/15/16) for another full Urinalysis.  I will look at the bill for more specific title of the test.  One interesting note is they said she has a tiny bladder (She weighs 9.5 pounds).  As far as the urine, all I can tell is it is yellow.  When it soaked through the comforter and sheets, it looked yellow on the white mattress cover and smelled normal.  It was very fresh (5 minutes) and did not have the ammonia smell yet.  When the test results come back, I will know about crystals. 

Litter Box:  I have one very large box (it the top rated/seller on Amazon) and I have had it for about two years.  I just have the one box.  I live in a condo with two cats and the place is a one bedroom and a lost upstairs.  The box is upstairs.  I use Ever Clean unscented litter.  I scoop the litter once a day, but have not cleaned the box and replaced the litter for a very long time.  Maybe that finally got to her.   The box is back against a wall on the upstairs loft and looks our across the condo.  She cannot see downstairs from inside the box.  Actually, if the door is closed, I would not think she could see much at all.  All this has been the same for her entire life.  I did replace smaller box that was covered without a door with a bigger one with a door about two years ago when she had her first accident.  Both covered. 

Keep in mind, this seems to be happening once a month and not every day.  Although I do keep an eye on her and bring her to the box when I think she might need to go.  She has never gone when I am away and never anywhere but the bet.  

My vet thinks she may like the bed because it gets cleaned whenever she goes on it.  

She actually sleeps on the bed.  I do have a comforter that has that synthetic stuffing (not down) and it is machine washable.  I think the washer is too small, but it does just fit and there is no residual soap after the load.  

I first use Rocco and Roxie I found on Amazon and then wash.  I turn the comforter around so the spot is somewhere else.  I also use the cleaner on the sheets and water proof mattress pad. 

I used a black light after washing and drying and it did  not show anything.  I hope that means I got it.  I do not smell anything myself even by pressing my nose into the material. 

That is where I am now.   I should hear back about tests in a week.  I think it might be behavioral as well.  

Do people really clean and replace all of the litter as often as web sites say? Once a week, monthly etc...