A couple things...
It could in fact be a PAC, termed a "non-conducted PAC". This is when there is a P wave which occurs earlier than its expected time (hence premature), not followed by a QRS (hence non-conducted). These are relatively common and relatively benign.
If the additional P wave is in time with when it is expected with no QRS, this is a second degree type II block. This is a more serious condition, as it often progresses to complete heart block.
The key to differentiating the above is whether or not this additional P wave is premature or not. If it is premature, and there is no QRS, it means the conduction system distal to the AV node is refractory from the previous beat which is why there was no QRS. If it is on time and there is no QRS, this is more serious as there is some sort of block in the conduction pathway.
The best leads for looking at P waves are your inferior leads which are II, III, and aVF. Since this did not appear in any of these leads, it is unlikely that it is any of the above. This makes me think it was the stupid u wave.
U Wave basic ECG patterns
Here's a site that explains more than you'll ever want to know about a u wave. Don't spend too much time on it though, the u wave is pretty low in specificity and doesn't really play a role clinically, at least in my experience.