Umpires yelling anything other than SAFE, OUT, or other calls that players need to hear is probably a bad mechanic. But regardless of the mechanic you and your partner decide to use, make SURE you are exactly on the same page regarding how you are going to do it - sounds like you thought you were going in, until it came time to actually do it.
Another reason this should be a look, and not a yell, is - once coach sees this once, if he hears you do it again and you (PU) DON'T have the call (perhaps there was other items more directly your responsibility that you were watching), coach will be irate that you "missed" the pulled foot (whether there was one or not!)
The mechanic that I, and my guys, use is for BU, when unsure about a possible pulled foot, to glance quickly at PU. If PU has seen a pulled foot, he's got his hands near his sides, pointing out (sort of a mini-safe-signal). If he either has an out, or did not see the play, he does nothing. This way, BU makes the call relatively seamlessly and coaches are not alerted to anything at all.