low coolant but no light

I had the coolant flushed/filled today and when I went out this afternoon to check the level, I took off the rad cap to see nothing!!! Seems like this would have tripped a dummy light on the dash, but it didn't. I went ahead and put some coolant in there so hopefully it'll be ok. Shouldn't I be able to see coolant in the radiator 'tube'? Also, if something is a miss here, what is the likely problem?

either an internal leak(intake gaskets and so on), or an external leak(could still be intake gaskets, among others. keep an eye on it to see if still losing coolant. also, did they bleed it correctly when flushed? air could have been the whole issue.

not sure if it was bled. mechanics are so iffy. I'll take note of the level this morning and see where it is tonight. I think they just didn't give it time to seeep down. I noticed when I filled it moments ago that I could hear it kinda trickling down the tube. There was no problem with leaking before I had it changed so hopefully intake is ok. thanks