Your terminology makes this hard to figure out.
First off we're talking about a S&W Model 28 revolver, right?
If so when I see "thumbpiece" I'm thinking you mean the cylinder release slide button. Yes? No? If "yes" then it's not in the grips. it's in the frame of the gun.
If I'm right and you're talking about the cylinder latch slide button then I have bad news for you. The slide bar inside the frame is supposed to have a threaded stud sticking out through the groove. Then the slide button along with an internally threaded "hollow" retaining nut fits onto this stud. USUALLY the hollow nut comes loose and the nut and thumb button leap for freedom. But it's also not uncommon for the stud on the slide bar to snap off and then the stud and the hollow nut along with the thumb piece go flying off into the dark. Since you're not seeing anything with threads in the slot it sure sounds like the threaded stud snapped off. If so then you should be able to see a small circular rough spot where it had been spot welded or silver soldered to the bar.
If I'm right and you don't see a stud on the slide bar in the slot for the cylinder release then you need to order the internal cylinder release slide bar, the hollow nut and the thumb slide button for your Model 28.