It sounds like you've already ruled out any medical cause for this change in Hoot's behavior, which should always be your first stop. Because you can trace the timing of the problem back to a stressful period in your lives when there were foster kittens around, it does seem likely that stress is weighing in here. First, try adding a third litter box; a good rule of thumb is to have at least one more box than you have cats. Make sure all your boxes are very clean (it might be worth it to toss the boxes you have and start fresh, as litter can carve grooves and scratches into a box over time that hold scent cats can smell even when you can't). Get the largest box you can — at least 11/2 times Hoot's length — but bigger is better. Consider using Cat Attract litter, which has herbs added and comes in both a complete litter or a shaker that can be added to another litter of your choosing. Make sure the litter box area is peaceful and not near any drafts or noisy appliances (such as an air conditioning or a washer that may change cycles suddenly) and consider adding aromatherapy to the room; there's a cat-specific product called Feliway, but lavender or chamomile can be as soothing to Hoot as it is to you. By easing the stress, we hope that Hoot will give a hoot about box manners again!