Now I've got your attention, your car should have four of these:
or something like it. That is a rubber cap for a brake bleed nipple, and it is more important than it looks.
If you don't fit them, water and other grot makes its way down the centre hole and into the seat. That makes them difficult to remove, because the hole, seat, bottom of the thread and the nipple rust together. Here's the one I eventually extracted from the rear caliper of my sister's Hyundai, with a £0.60 new one:
You can clearly see the marks from getting out(I clamped it in the vice and worked the caliper to and fro to get it moving, finishing off with some mole grips), the grot at the working end and the damaged threads.
This wasn't the reason I removed the caliper from the car; that was to free off the piston and handbrake mech. It
is the reason I've just ordered a new caliper, because the seat the nipple is supposed to seal against and the mating threads are equally grotty and it leaks like the Titanic
While there are some things I could have attempted to rescue this, the rest of the caliper isn't brilliant and isn't worth the time when a new one is £66 delivered.
So to the point:
If yours are missing, I would recommend you get some and fit new bleed nipples while you're there. That's less than £1 per caliper and could save you a lot of grief and expense.