Player hiding his rolls from me

Figured the DM’ing flair fit better as there hasn’t been a table dispute *yet*.

Okay, first time poster here, but been lurking for a few years. Just ran a session for a group of friends a few days ago, and between the chaos of the session, some pain I was dealing with and some other things, I just plain forgot this occurred until an hour ago.

So it’s five players, all good friends of mine that I’ve known for years, some I’ve known from before I ever knew dnd was a thing. They’re all level 4, and they’re helping defend a town from being sieged by goblins, some undead, etc etc. It’s during this fight that I ask a few players for a constitution saving throw (I don’t remember what triggered, but the DC wasn’t super crazy, like 14 or 15.)

Well, the players roll, and I ask what they rolled, and get 3 of the 4 affected players answers. The fourth player asks what the DC is. I ask what he rolled, and glance down to see that’s he’s covering his dice tray. He doesn’t tell me, instead asking what the dc is again.

Something about it seems fishy to me, but I’m not in the mood for confrontation (pain from work and general irritation) so I just say it and move on and lo and behold, he tells me like, 21 for his roll or something (he has a high Con, I wasn't surprised). But looking back, like, am I in the right for thinking that was weird and off, or am I just overthinking things? I mean, I don't know what enemies you can throw at a level 4 party that requires over a DC 20 saving throw to beat their spells/effects/whatever. (I mean, I guess technically as a DM I could throw a tarrasque at them, but you get what I mean)

Anyways, I do plan on talking to them, just wanted to get some second opinions first.