I was once the victim of the opposite side of this coin – I didn’t say I *wasn’t* doing something, and that got me killed.
AD&D1: Our party was invited into the back room of a tavern to have wine with a shady group wanting to congratulate us for a job well done. The wine glasses were filled, and the DM assumed we all drank. I, though, was leery of the set up, and was not going to drink the wine offered. The DM didn’t ask us or clarify or even give us a moment to say anything about drinking. He just went from “He makes a toast to your good work, and everyone drinks. Give me a saving throw versus poison.”
I was trying to think of a way to get rid of the wine in my glass without anyone noticing that I didn’t even register that second sentence until I heard all the other Players moaning and cursing. Then I said, “Wait a minute! I didn’t drink it.” But, as you can guess, that was taken as a rather unsportsmanlike statement. The poison was the type to only do hit point damage, and was intended to just weaken us all, but my elf magic-user didn’t have many hit points and would have died even with a passed save (less damage than a failed save).
The DM, being a good friend, and taking pity on me, kind of twisted events to have a cleric near by that came to neutralize the poison before it killed my character.
Because of that situation, about 20 years ago, and because of scenarios like in the OP, I make sure that 1- my trap set ups are unmistakable, and 2- I’m sure of the PC’s action, before “springing” a trap.
For instance, in the above scenario, the poison could easily be slow acting; allowing me plenty of time to make sure the PCs had drank the wine before asking for saves. And in the OP scenario, I wouldn’t be so anxious to spring the trap that I couldn’t wait for someone to *explicitly* trip it (or even bypass it with a smart trick).
Quasqueton