Perhaps, they may be best classified as 'mathematical folklore'.
Let me suggest that citing it as 'mathematical folklore' would be incorrect. Based on what you have said in the question, you have proven the result (you know it to be true) and you also are not aware of a proof in the literature, although you believe one exists. However attributing a result to folklore typically implies much more than that: that the result is well-known by many people in the field, but that it doesn't have a canonical published proof in the literature. As you aren't an expert in the literature in this area, these statements go beyond what you can claim; you would need to be aware, and not just suspect, that the result is common knowledge.
I believe that if I spend enough time going through articles in minor journals/theses in theoretical CS/applied mathematics, eventually, I will find the statements/proofs of these results.
A common scenario in applied research! My approach in such situations is to somewhat "downplay" the result; for example, don't state it as a theorem, but as a proposition.
And don't claim in your introduction or your list of contributions that you have proven a new result; focus on the new application instead, and the theorems are just there for completeness of the formal development or out of necessity. Finally, depending on how much effort you (or a coauthor) has put into searching the literature, either say that it is not known to your knowledge, or that it may be known, but you include a proof here anyway.
If you do all this and word it carefully, I don't think you are crossing any ethical lines by not citing the result. And you are certainly not committing plagiarism just by not being aware of something.