Dog using litterbox?

I have a very strong suspicion that one of our dogs has been using Leiki's litterbox to do her business in.

When we take Sadie outside to go to the bathroom (every 4 hours or so), she almost always pees but will rarely poop. We'll go a few days without seeing her poop, but I can palpate her and know that she has gone... somewhere. I spent days frantically walking around our apartment trying to find out where she's been going but also noticed that when I would clean the litterbox out there would be big poops mixed in with Leiki's normal sized cat poops. Leiki is completely fine, I've been scooping it right after I see her go in to make sure that she is going and that her poops and pees are normal - and they are - then these mystery poops will show up when I'm fairly certain she hasn't been in there (like when she's been sleeping for hours on the dog bed right after I cleaned her box). They are also smellier than her poop are, so I can tell when the mystery pooper has gone because you can smell it when you walk by.

Sadie was litterbox trained at one point, but I stopped using it (and pee pads) because I was like why would I train my dog to poop in a litterbox and then have to clean it out, if I wanted to do that then I'd just get a cat (which we eventually did, lol). So I really think it's her. The problem is that she's so sneaky about it that I can't catch her doing it. She'll be laying with us or in her dog bed one minute, then you look over and she's not there anymore and god only knows how long she's been gone. We call her and she comes sneaking back in the room. Without physically leashing her it's hard to tell. I've been thinking about setting up a wifi camera there to see if she is going in, but I don't really want to have to sift though hours of litterbox tv to see.

I don't have a problem with it, but I wanted to know if it was dangerous for either the dog or the cat. Neither of them seem to mind, although Sadie will occasionally snack in there, but Leiki is still using it fine and this has been going on for about a week.

I don't think it's dangerous to either, the only potential problem I can see is the cat being upset because the litterbox is not as clean as it was before, but adding a few more litterboxes and/or scooping more often would take care of that. And of course both animals need to be wormed regularly to prevent passing worms to each other.

I have never owned dogs so I don't know much about their physiology but it's often made me wonder how healthy it is for them to hold it until they are allowed to go out. Even if it doesn't pose health risks I am sure it is bothersome (I know it would be for me, lol) so good on Sadie for figuring out where to do it when she feels the urge!

"although Sadie will occasionally snack in there" - unless I'm misinterpreting this, then you DO need to worry. Litter can clump up, block and kill animals who eat it...

Maybe if you put a cover on the litter box? I don't know how big the dog is, but maybe a cover would prevent the dog from getting in, and any "snacking." I think is a fairly common problem. . .our dog ripped open the cat poo bag once and dragged it all over the yard (probably ate some too). She was fine, we were just disturbed.

Also you could try taking your dog outside right after she eats, they usually poo right after they eat.

Sadie is very... unique, lol. She's a very weird dog. If she has to go out to go pee/poop she'll ring her bell on the door, or if we're not in the living room she'll come in whatever room we're in, bark, and then take off to the door - repeating as needed until we get up and let her out. We take her out whenever she has to go because she has had a lot of problems with UTIs and had surgery in October for bladder stones. Our dogs are also older (Sadie will be 5 in October and Fynn will be 3 in August) so they don't go within 1/2 hour of eating like a puppy would.

We do not use clumping litter, so that really isn't an issue, and we use pine litter so there isn't really any dust and if she does ingest any it's basically the equivalent of her chewing on a stick (yes, she will eat sticks).

Both dogs are wormed monthly, they haven't had any worms since we got Fynn a few years ago. The cat had worms when we first got her, but she's also worm free now. My boyfriend makes fun of me because I break up poop when I clean the litterbox or when they go outside to check for worms and whatnot - we also do a fecal float twice a year. I'm pretty paranoid about worms.

The litterbox is covered, but Sadie is about the same height as Leiki, although a bit longer. She doesn't snack much - maybe once a month if that. I clean the litterbox whenever I see Leiki go in, right when I get up in the morning and when I get home from work or if we've been out. She doesn't seem to mind a dirty box, but it never has more than 1 poo/pee in it at a time because I do clean it so often. She has actually gone in it and used it after Sadie's been in there since I posted this, I had a nap the other day with Sadie and Fynn on the couch after work, when I woke up there was a cat pee, poop and a dog poop in there.

Maybe it's territorial marking? Before my sister took our other cat, he would go and pee in the litterbox immediately after we cleaned it and after Lucky went, which made me think he was marking his territory. So maybe that is what your dog is doing? The only other thing I can think of is to elevate it off the floor so the dog can't reach.