Strong odour on bed

I'm really new to this site so apologies if this has already been asked. I have 3 cats and one of them has started to leave a really strong odour on my bed. I'm pretty sure it's the youngest female Leyla, who is 16 months old. She's been spayed and is an indoor cat. She's the one that usually sleeps on the bed with me which is why I think it's her. In the last week I've been in the bedroom twice when there's been no smell and then half an hour later it's quite strong smelling. There's no stains or wet patches on the bedding and I've actually had to sniff around to be sure I wasn't just being paranoid. I don't think it's urine but I'm baffled as to what it is. Any advice is appreciated. Thank you

Is the odour like stale urine?  If so, it is most likely that one of your cats is urinating on the bed even though you are not seeing any staining, possibly because of some stress and seeking reassurance or not being well.  You  can find spotting or inappropriate urinating with bladder inflammation and cystitis among other things.  If it is more of a fishy smell it could be anal gland secretions.  You may or may not see one of your cats ' scooting' on the floor which can be a sign of this but it doesn't always happen.

It is always difficult when you have more than one cat and have to find a way to figure out which one / ones are having the issue, but do what you can to trace it.  That way you can limit the stress (and costs) of vet visits to a minimum.   I hope this is helpful in some way, and that you can work out which cat this is happening with.