Actually, I agree with v3xt. In real life, we wouldn't be able to see the sentry, all we'd know is that suddenly some of the drillers on a sub were shot. I think nothing should give away the position of the sentry, not even the time machine, unless the outpost is in your sonar range...And players should get notified that their sub is being targeted by a sentry(ies).
You could, however, work out where the sentry is located and the range of the sonar if you blindly send 3 subs at where you think the sentry is located. If you time it carefully and send enough (and the same amount of) drillers (and are lucky and smart) there should be 3 distinct points where the subs are shot, if you match the 3 points up you will be able to find the sonar of the sentry (as a circle can be defined by any 3 points which it passes through). And finding the circle can help you locate where the sentry is and and range of the outpost that the sentry is at. It would be tough work, but that would mean that smarter people would be able to get more of an advantage by knowing where the sentries are while other people might not)
Kings aren't OP
"Imagination is more important than knowledge"
----Albert Einstein