Hi Shadow! I'm no vet, but this renewed 'marking' may not be that. Our cat Percy suddenly started having the odd 'accident', which culminated on his weeing on our bed (yuk!, wasn't my side though!!
). A few days later he came in one evening in a terrible state - yowling in pain, and hunched walking. Rushed him to the vet, and it turned out he had Feline Urinary Tract Disease. Basically, he forms crystals in his bladder, which to him feel like cut glass when he tries to wee. It's so painful for them to 'go' that they don't, until it's forced out. Percy had got to the stage when the urine had become so full of crystals that he couldn't pass it. It's more common in overweight male cats (Percy is huge!). He had several days in the vet's exclusive cat hospital (megga bucks!). He's now on a prescription diet for life (more megga bucks!).
I would be inclined to get a vet's appointment ASAP. Keep your cat in overnight beforehand, with no litter tray, to make sure he has a full bladder when you take him. I must admit, we thought Percy was just being really naughty. The vet said that if we had left him until morning, we may well have lost him, as his kidneys and other organs would have started to fail.
I do hope that's not the problem with Shadow. If it is, then it's treatable, if it's not, then the vet can check for other things. Good luck, and let us know how you get on.