I do understand that some people like to be afraid of the tax authorities, because "they have the bigger stick". But in reality they operate within the rules that have been set for them, and are pretty far from being the allpowerfull know-it-alls some (and sometimes they themselves) would like you to believe them to be.

Let's put us in their prospective, and let's look at something as straightforward as a rent extremely closely. What can you check?
1. Can the examined person prove he really rented a storage room? - Yes, he can show a wire transfer from his/her own bank account to the storage room providers account as proof that you can check.
2. Could the person sublet the storage room for profit? - Yes, if the storage room provider doesn't forbid subletting. If you as the taxman are being  extremely thorough you contact the storage room provider and ask if subletting is allowed, and to further back nr.1 if they can confirm that they have the examined person as a client. The answer is yes for both.
3. Can you as the taxman find anything that would indicate that the income that was declared to be received as rent was in fact received for something else in reality? - No you can't, as you can't trace where bitcoin or crypto transfers originate from. Plus everything else that needs to be in place for the declaration to be true is in place (the storage room is really rented by the examined person and it is possible to sublet for profit).

There isn't anything for you as the tax man to further investigate or documents you can request when we are talking about a private person being examined.