Sea Essay is fitted with an AIS transponder. Her last recorded position can be viewed using a number of external websites. All of the sites mentioned below seem to be reliant on signals recorded and reported by volunteers living in coastal areas and coverage varies hugely and is not guaranteed. Consequently, if the vessel you are tracking is not visible one day then that is not necessarily a cause to be alarmed. Note also that the track is sampled and is not necessarily the exact track the vessel followed.
Eg For example using vesselfinder.com. To see where Sea Essay has been in the previous 24 hours look for the following on the screen and
MarineTraffic.com provides a similar service with a ~3 day track history for free.
ShipAIS is an alternative service for the UK only. It is also free to use and has a much longer archive / memory so you can go back months if necessary.
Shipfinder is another one with up to 5 days history but coverage is not as good as MarineTraffic.com or vesselfinder
Please let me know if you are aware of other services. Especially any that display a longer historical record for free or which allow a data download.