I don't think Bruce had actually been a recluse for the entirety of that 8 years. 5 years into that timeframe, he was pouring money and resources into the fusion reactor project. Surely that would've required him to maintain at least some sort of public presence, at the very least at Wayne Enterprises if nowhere else. After a second viewing, my impression became that he was only a recluse during the last 3 years prior to TDKR, and that the need to shutter the reactor project compounded his grief over Rachel, driving him into seclusion. I don't remember her exact dialogue now, but during the very first conversation between Bruce and Talia-as-Miranda, she made an observation about the reactor project and his seclusion that implied he didn't vanish until after the project had been shut down.