I will go one step farther
You have described a very common conditoin. The Kalsbeeks covered one possible cause (What I call the dog shower, or outside shower) is often the culprit.
But in this case you said something else noteworthy "STORAGE"
Did you bypass the water heater before putting it in storage (many do)
And do you have the 3 valve system
DID you remember to CLOSE the bypass valve?
The third option is like the first (Outside shower) only it's the INSIDE shower.
Yup, those are #1, #2 and #3 on the hit list for this issue.. And unless you have a 10 gallon in a six gallon hat water heater (Attwood makes one like that) There is no suspect #4
(If you have that 10 gallon in a six gallon hat attwood the mixing valve that does the magic is also suspect, but odds are you don't have it)