G-019: Crystal Coast Resort, Ranulf’s On-The-Water, The Tank

Just an fyi, the relevant GURPS skill for tailing someone is Shadowing.  It has defaults to IQ, Observation (which Lia has and is used for gathering details about an area or routine) and Stealth (which Oliver has and is used for sneaking and hiding).  

Observation can also be used for static surveillance.  Depending on whether any technology is being employed, Electronics Operation (Surveillance) (which, again, Oliver has) may also be useful. 

To complete the circle, Observation defaults to PER and Stealth defaults to DX or IQ (one of few skills that do so).

It may also be worth noting that Lia has difficulty blending into crowds.