We have a TNR we care for named Hitch. We had him TNRd in December and he received the rabies and distemper vacs. We ve only recenly been able to pet him but when we do he purrs and drools. I've watched him but he does not drool when not engaged with us. Our indoor cat Psycho does not do this and I've not ever had a cat that did. Is this something that cats sometimes do?