One way of preventing the others from discovering his movements could be to leave behind a Shadow Clone while he goes out to work. After all, Madara's Kage Bunshins were so effective Tobirama couldn't distinguish it from the real body even after prolonged interaction with it. Certainly Orochimaru at full power (and then some) could create one that could fool the likes of Hebi and Yamato. Or, he could create a subservient genetic clone of himself or something like that.
Also, I don't think Sasuke, or anyone for that matter, pops in on him at all. Suigetsu notes that it's been a long time when he last saw Sasuke, and Orochimaru is pleasantly surprised when Sasuke dropped by. A very stupid move on Konoha's part, but whatever. To add to that, Orochimaru can successfully avoid Konoha, Suna, and Akatsyki and has been for years. Odds are, he can easily avoid Yamato.
I disagree about right now being a time to act, actually. It would be better, I imagine, to wait until Naruto and especially Sasuke are dead. After all, we don't know where Hebi's true loyalty lies, though it's most likely with Sasuke. They could be there to keep an eye on Orochimaru for Sasuke. But that's lead back to my point about Kage Bunshins and genetic clones.
Though, yes, amassing followers would take time and effort. Still, he'd only need a few semi competent ones who know how to keep quiet and lay low. Like the Death Eaters in Harry Potter lol.
