Hartley has been linked with a move to Greek side Iraklis Thessaloniki and Hearts.
Vennegoor of Hesselink has been linked with a return to PSV Eindhoven.
“I had a good long conversation with Paul. He came in to see me.” Mowbray told BBC Sport.
“The outcome of that is Paul won’t be offered a new contract. He is free to look for other employment.
“I have had no contact with Jan. Jan is out of contract – he is free to seek other employment and I’m pretty sure that’s what he will be doing.”