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​That's a good question, and you're right: There are situations in which san, like sìk, can be omitted. Here's a little illustrative dialog that occurred to me:

A.   Ngeyä sa'nokìl poltxe paylì'ut? 
B1. Poltxe san frawzo sìk, slä oel poet ke spaw.

That would be the standard way of answering, but in conversational contexts, B2 and B3 would both be allowable:

B2. San frawzo sìk, slä oel poet ke spaw.
B3. Frawzo sìk, slä oel poet ke spaw.

B3 is acceptable as long as it's clear from the context that B begins by stating what the mother said and not commenting on his own. It also helps if the quote isn't too long, so that the sìk comes along relatively quickly to further clarify that it's a quote.

In thinking about what happens in English, I realize the same kind of situation can occur:

C: What did your mother say?
D1: She said everything's OK, but I don't believe her.

That, like B1, would be the standard response. But what if D answered like this:

D3: "Everything's OK," but I don't believe her.

If D simply responded this way, would it be clear to C that D was quoting the mother rather than stating his/her own conclusion? Perhaps, but I'm not sure! In this case, Na'vi is less ambiguous than English, since the sìk is obligatory, given that B's response continues beyond the quote with a personal comment.

Hope that helps!