Note: 1 long blast = 5 or 6 seconds 1 short blast = 1 or 2 seconds Three short blasts means "My engines are going Astern". (It doesn't necessarily mean that the ship itself is going astern).
One long blast can mean one of many things:
1) I am turning to starboard2) In a situation of restricted view (ie turning a blind corner on a river) it's a warning signal to warn other vessels of...
At Anchor, It is used addition to the required rapid ringing of the bell for five seconds (not to exceed one minute interval) to let other vessels know her location.
Continuous soundng of a ship's whistle is a distress signal. If you only heard one or two blasts the ship could be signalling as required under the rules of the road.