Example:
The Capsuleer Log-Off Timer is activated when an offensive module is used on another player (including non-targeted weapons like Smartbombs or ECM Bursts) or the ship is being attacked by another player in any capacity. This timer lasts for fifteen minutes , and affected ships will remain in space for a full 15 minutes after logging off and completing any ongoing warp. The timer will reset on each attack of the affected ship, independently of the pilot being online or not. This means a ship under attack will remain in space until the attacks on it have stopped fully more than 15 minutes.
Doesn’t even have to be an actual log off timer. It takes about 30 seconds for your ship to disappear from the system and during that time it remains visible on dscan if you are in range. If it lit a trace, you would see the trace remain on dscan and not the ship. That Tengu just logged off in space (no safe log off).
He could have also disconnected while having a timer as well. In any case this is a great opportunity to bust out the combat probes and get an easy kill.