Found this a bit strange as I've never seen this before in SHIII. It's not all that important to me but thought I'd let Jaeson and/or the Devs know about it.
At the end of my 1st patrol I went into Vigo Spain to resupply from the Bessel. My career start was Wilhelmshaven Aug 3,1940. After Sept'40 subs are transferred to Lorient. Being with the Bessel from Aug 23rd to Sept 15th for repairs, paint and resupply I started my 2nd patrol which said return to Lorient, as expected. But the above pops up in SHIII Cmdr 2.7. I click Ok and continue on with a Null patrol. I'd changed the Null to BE66 but it didn't stay that way. I spent a week at sea sinking one Merchant and missing a large Convoy. Saying the heck with it, I head for Lorient and spend 7 days in base. Now on my 3rd patrol I keep getting the same mssg everytime I start. That goes for all saves while in this 3rd patrol too. Is this a bug with SHIII or SHIII Cmdr?