So people are annoyed about thieves hiding in structures. Thieves are annoyed about being randomly marked by sentries and towers in half the map. The solution doesn't currently seem to be pleasing anyone.
So here's a suggestion. Take away tower sentry field upgrade, and replace it with a tactivator activated upgrade that gives marked in the area for 5 mins on a 10 min cooldown (or whatever) like when a keep is just capped. Give this upgrade to keeps also, and make it one of the early upgrades for both towers and keeps.
With this change, unmanned towers become less obnoxious to fight around for stealth classes, but players can now selectively use marked tactic to flush out players hiding when needed without needing to repeatedly nerf said classes or introduce more stupid mechanics. Making it a T1 upgrade means the hiding players have a short window to do their thing, so it's not a guaranteed win, but you're not gonna be hunting them for the next three hours because they'll be forced to leave when the tactivator is used. It also promotes active scouting, as you don't just get a map marker on the enemy zerg for having an upgraded tower, while sentries still retain that functionality since that's the entire point of them being there.
Critical Kit, Deadeye.
“If you find yourself in a fair fight, your tactics suck.” - John Steinbeck