Some context: I'm a newbie DM running a campaign which requires the party to do a lot of scouting and infiltration missions. Especially in buildings. It's a city campaign happening in Waterdeep so I expected the party to do a lot of lock picking, disguise self and other similar standard "stealthy and rogueish" stuff.
Here's what's happened: the party came up with an idea: why don't we just cast See Invisibility on one of the casters, then Invisibility and then Gaseous Form and then Nondetection, all on the party's monk. So, instead of party's ranger doing some smart infiltration stuff in the Lord's mansion at midnight, we got a monk in a form of gas cloud, easily entering people's homes through the chimneys like Santa Claus 😆🎅, then going from a locked room to another locked room through the keyholes, looking at everything, listening to what everyone is talking about in every room, all in broad daylight...
And all that while not being detected at all. Because how could you reasonably detect such a thing?
- Is it visible? Definitely not. Even if there are some "buffed guards" with See Invisibility, it wouldn't work because of the Nondetection spell.
- Does the cloud smell? No, the spell doesn't say that.
- Can people hear the gas "moving"? I assume also no, because how in the world could you "hear" a slowly moving fog?
- Could the infiltrating person in a gas form throw badly on the Stealth check and accidentally tip that vase on the table? No, because the gas "can't manipulate objects".
- Maybe... A creature with blindsight could detect this sort of gas with a high enough perception check. But as I've said, this is a city campaign, so all guards in a mansion having blindsight may not sound reasonable...
Am I missing something here? Or should I just give up on trying to make all of the scouting / infiltration encounter challenging and admit that my party came with with an ultimate scouting / infiltration combo that just makes these encounters so easy? Based on your DM-ing experience, what you'd suggest to do to make the use of this combo more challenging?