@pemerton is the authority here, his OP said no. I'd expect there are possible approaches, like saves, or something, but it is unlikely to be a clear process. Part of the reason for this is that games like RM or AD&D adjudicate action and situation, and there's no way to make such a game complete.
Well, there is. Here it would involve breaking the scene down into units of game time (maybe each a few minutes long) and determining for each time unit a) what the PCs are doing and-or how successfully they're doing it, and b) what the NPCs are doing and how successful they are.
Determining what the PCs are doing is easy: the players will tell you, and resolution runs as normal.
Determing what the NPCs are doing, in a more granular way than just "searching", would I think come down to some random dice rolls, as (if I'm reading the OP scenario right) the searchers didn't have much to go on other than suspicion, and thus their finding or noticing anything amiss would pretty much come down to sheer luck. Roll the dice.
As for the use of spells, the OP indicates (but doesn't say outright) whether the NPCs' spells-available were written up in advance. If the players caught the DM off-guard with a sudden decision to teleport to the ruins and search there, the NPCs' spells etc. not being pre-written is completely understandable: the DM is in full-on react mode, and would have to quickly set some odds and then roll to see if a) any of the NPCs had
Detect Magic available (or indeed if any of them were casters at all), and then b) the RM equivalent of an Int or Wis check to see if such spell got cast.
And if the riders do notice something amiss, it's then on the GM to play them true to themselves as to what they do next. Do they take up defensive positions to attack anything coming out of the ruins (i.e. the cautious approach)? Do they send some riders to get help (the delay-till-later-and-hope-the-problem-goes-away approach)? Do they try to bust through the hollow floor (the aggressive approach)? Again, if these are NPCs the GM has just had to dream up on the fly, some dice-rolling - maybe along the lines of a sliding-scale morale check with lower meaning more cautious and higher meaning more aggressive - might again be in order. I've done things like this many a time in the past when I've been unexpectedly put into wing-it mode.
4e adds a way to adjudicate in a goal centered way, so it can be applied to basically any meaningful situation.
If you're thinking of skill challenges, I'm not sure here. You'd sacrifice a huge amount of granularity (and drama) in order to play this all out as a skill challenge...and would it be two opposing SCs - the PCs get one for their searching of the ruins while the NPCs get one for their searching for the PCs? From what little I know about 4e I don't think NPCs get to use skill challenges, so by RAW that option is out.
Never mind that if the PCs fail on their SC it still doesn't necessarily mean the riders have found them - the failed SC on the ruins search could be for any number of other reasons e.g. an impassable barrier, a pit-wall collapse, they simply don't find anything of use, or whatever; which kind of leaves the riders hanging even though they represent the dramatic element.