My first thought would be to check the litter box actually. Did the lovely lady who came to feed them also keep their box(s) clean? Did she run out of litter and buy a different brand from what you usually use? Cats can be very finicky about this - understandably so, I'm a bit picky about my toilet paper as well, and I don't have to lick my paws clean after using it - and it's a common reason for cats to relieve themselves in inappropriate places. Fortunately, this is usually a fairly easy fix.
Another thing with the suit case, it could be territorial in that case. Were there other cats (or any pets) where you were when you were away? Maybe you had the scent of another cat on your clothes or in / on your suitcase (my s-i-l cat LOVES suitcases, and made herself quite at home in mine when I visited her and my brother), Loki may be reclaiming the suitcase as his. This is less easy to fix, but if Loki is not neutered, this sort of behavior usually calms down a bit afterward.
Finally, the potentially more serious possibilities are medical. If Loki is having trouble urinating, he may associate his box with the pain from urination, so he's going someplace else. This is also where separation anxiety comes in too, because some cats develop bladder problems because they're anxious / stressed. This is less about actual "I'm mad at you and you're going to pay" than it is about "owie, owie, my pee hurts and I suspect it's the box because it always hurts there", even though the initial trigger may have been "where is mom / who is this lady?"
So, best thing is to do two things: Check his litter to make sure everything is as it should be, and take him to the vet to be checked that his bladder is in good shape.
Keeping my fingers crossed for you both! <3