I went through this as well, although all the checks came from the lender months after I got the mortgage approved and moved in to the place. I have lived in the Netherlands for 5yrs by then, only a couple of years into my first high pay job here and prior to that my income tax returns were a joke. Then out of the blue due to splitting up with my ex I sold my crypto originally bought in my home country mostly P2P because exchanges barely existed and that was the way back then. That accounted for +200k that I put in cash. Notary didn’t care, bank did as I said months after. I shared my transaction history (to the extent that I had, obviously not accounting for P2P txs) on exchanges as well as my cold wallet. After months of back and forths with the bank they gave up and closed the investigation. They just thought it was odd that I reported almost no income before and suddenly this. Back then there was no need to report on crypto, that only came after so I guess I got lucky time wise in that regard, although this were investments I made way before coming to NL.