The issue is that while the police now seem to think Gaia has come to harm, and may be dead, that was not something they would necessarily assume from the start, especially given what was known early on about her mental state and physical health. Surveillance could have been undertaken very early on if (a) there are viable suspects/PoIs and (b) they think she might be alive and being held somewhere, and there's a chance the suspects may lead them to her or other evidence. The possibility that she is alive and at risk might well make surveillance a greater priority than in another case with different circumstances.
It's a question of balancing probabilities and deciding what would be the more productive use of personnel.