When performing an Investigation check to investigate traps, do you activate the traps if you roll under the DC?
If you are, for instance, checking out the floor looking for pressure plates and roll low, would you activate the trap? Or would the DM just tell you that you found no traps?
This is up to the DM and how they have decided to implement how players go about with their investigation checks. Some checks have no chance of triggering a trap and hence don't apply to this situation, but there are a few situations where that might not be the case.
For example, if you were to check all the tiles on the floor of a room for a pressure plate you could accidentally trigger the pressure during your check as you might step on a trap during your check.
(If you rolled low enough to miss the trap)
In my experience in such cases the DM often asks how you go about doing the check and depending on the roll (if it's low enough) and your actions, you might inadvertently trigger the trap.
As iaminsensible's answer points out, typically failed Investigation checks to find traps in areas with traps simply result in no traps being found, as though you had rolled to Investigate an area that really didn't have any traps.
However, there has been a precedent set for failed attempts to disarm traps resulting in activating the trap, usually if you roll too low (say, the DC is 15, and you roll a total of below 10).
This option is also presented in Xanathar's Guide to Everything; some sample traps on page 114 includes the Crossbow Trap, which has the following:
Countermeasures. A successful DC 15 Wisdom (Perception) check reveals the trip wire. A successful DC 15 Dexterity check using thieves' tools disables the trip wire, and a check with a total of 5 or lower triggers the trap.
This is also further described on page 117 under Disarming a Simple Trap (the last paragraph).
This suggests that if anything was going to trigger a trap as a result of rolling too low, it would more likely be an attempt to disable it rather than to find it through Investigation (or Perception, alternatively). However, ultimately, the consequence of a failed skill check is up to the DM.
Investigation check means usually scanning the area for traps so if someone were to not succeed it means that they found no traps. But ultimately it is up to the DM.
