Aged parenting problems

I have a glimpse of the future!!

Oh my ...

Finally!
Pair of knickers behind (well in between the panels of) a doubly radiator.
Now for the awkward conversation and a trip to the doctors and a gift of Tena pants. Hopefully its just a UTI.

Urine glows under black light. You can get a black light flashlight on Amazon (and elsewhere) which you can use to locate any urine spots to treat with the aforementioned WeeAway.

You can get a spray called something like WeeAway (others probably available!) from one of those online/catalogue stores. It's supposed to work for both people and animals, I think.

I bought some when FiL was poorly and it seemed to work well.

Your MIL sounds marvellous. Hope you find the source of the smell soon. It could be she's hidden some wet clothing because of embarrassment in the way a child might. I don't mean to be unkind when I write this.

I would have thought you would have noticed if the GG was incontinent. This may be a long shot, but electrical wiring can smell very strongly if it is overheated. Check lamps, overhead lights etc. Sometimes a bulb that is too high wattage for a lamp will smell strongly. I’m assuming you have checked bathmats, towels, around the loo, and checked bedding and mattresses, checked cut flowers and pot plants. I hope it clears up soon!

Maybe GGG (that's some feat!) isn't the source of the stink.

Did any smell of urine emanate from her during the visit? I'm sure you would have noticed

Maybe check if any wet clothing has been hidden somewhere, behind radiators , under furniture.

Try wiping surfaces with a mixture of white vinegar and bicarbonate of soda. The vinegar smell goes quite quickly. Sprinkle bicarbonate of soda on to the carpets and soft furnishings, leave for 30 mins and hoover up.
There are proprietary sprays that people use when training puppies, I don’t know if they’ll work.