Please do not discount a discharging battery. S-Types are particularly susceptible to loss of battery charge. Mine will start doing sill things with codes etc if it is not kept regularly charged, even if being used on long runs. I have a CTEK 7.0 and make a point of having it charge the battery at least for 24 hours once per week. The battery is a 95Amp.Hr as recommended and is just 12 months old. I now do not suffer anywhere near as many silly codes and warning cropping up.
The submission concerning hoses is also very important, if in any doubt run or have run, a smoke test which will usually show up where there is any potential leak. In the "good old days", (excuse that but I am now 72 years young), we used to very sparingly use carburettor aerosol cleaner around the hose joints and listen for any increase in RPM. That would also give a good clue where the leaky hose was.
Good luck with correcing the proble. Best Wishes and Regards, John