While I don't subscribe to GP's view that satellites have bandwidth for "video recording the entire planet at all times", it seems plausible that SIGINT satellites would have a relatively constant bandwidth available for observation. As a (non-geostationary) satellite passes over relatively unpopulated areas (e.g., the southern Indian Ocean), it will be "bored" and thus far more likely to notice a single low-power transmission.
On the other hand, if it is storage and downlink bandwidth that is limited, sats would probably be programmed to ignore or discard uninteresting signals. This seems likely to me.
So the question may be whether commercial airliner pings from the southern Indian Ocean area were considered 'interesting' at the time and, if not, did anyone realize there was a rogue 777 in time to re-task the satellites?
Regardless, I'd bet those signals are considered interesting going forward.