Recently I took my almost 7 year old male to an agility trial (trying to get him used to new places so he can be more calm). As we were waiting in line for our turn, he was actively working the nose, standing on his hind legs trying to find whatever. Didn't think anything of it really although he hasn't exhibited this kind of behavior since he was neutered years ago. Then once we started our run, it took a turn back towards the line-up area. Off he goes to the fencing and again stands on hind legs working the nose and trying to find whatever female must have been coming into heat (females in heat are not allowed in the trial); wasted lots of time trying to redirect him back to me. Is this return to old ways common in older males?
It may have been a bitch coming into or out of heat. It may be a bitch with a UTI, which many males read as heat. It could be something else entirely. If you can find people who own bitches in heat, ask them to meet up and do some intensive "leave it" work with him.
Neutering helps, it doesn't always stop or end the interest. Obviously if it was a bitch, she was putting out irresistible odors. Poor "Casanova"... sometimes the brain forgets.