It is very rare for a viral infection to recur in a recovered patient, since recovery implies that the patient has developed effective antibodies and is resistant to further infection.
A huge exception to this case is HIV infections, where the immune system itself is under attack.
This said, viruses can and do mutate rapidly owing to their very primitive and error prone replication system. In HIV, where the immune system is itself compromised, and where non compliance issues (skipping doses of antiviral drugs) a number of mutations have been shown to arise, and patients can develop drug resistant strains.
There are, in some disease models, cases where "reservoirs" are not accessible to antibodies, but they are not, to my understanding, typically viral diseases. This scenario is well known in some types of cancer diseases, where cancer cells are localized in an area to which a treatment drug is not able to penetrate.
I have read papers indicating that the virus may have originated in pangolins - ant eaters - and became "humanized" when the virus developed in humans. I have a post in this section discussing this scenario, but it may be, as you say, "spaghetti."
The operative point here is that if you know someone who has recovered from Covid-19 it may not be safe to assume that they cannot infect a person.
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