You're correct in that they should have checked his visor because it had been tampered with in the past, but they still didn't know for a fact that it had been tampered with. I had always surmised that his visor was taken away so Geordi simply wouldn't be able to see not that Soran was going to tamper with it. I mean in the movie Soran tells the Klingons to "eliminate them." At that point, I don't see why Soran would have tampered with the visor just to then destroy the enterprise if that old bird of prey could. To me, he seemed generally interested in the visor as a doctor so he studied it to see how it works...etc but at the last moment, he decided to do whatever it is he did with it, then send Geordi back. But again you are very spot on. They really should have checked it. I do stand corrected.
Oh, I agree that they wouldn't necessarily assume someone had tampered with it. Geordi obviously thought that Soran had just taken the VISOR to deprive him of his sight as a part of torturing him. The only thing that sort of hints that they've done something to the VISOR was when Soran says to the Duras sisters, "I think it's time we gave Mr. LaForge his sight back." But, of course, that's just very vague foreshadowing to us in the audience and not something Geordi heard.
My point was just that given what happened in "The Mind's Eye," I'd think they'd have protocols in place to be extra super careful about Geordi's VISOR. Like scanning it everytime he's been off the ship. Or at least scanning it each time it's been in someone else's hands. We could argue that perhaps Geordi just didn't think to mention to anyone that Soran had taken his VISOR for a while, but again you'd think that after what happened before, he'd be instructed to always report something like that.
Bottom line, I can't think of any way to justify them NOT checking his VISOR other than that somebody got careless.