In case you don't want to or can't read the article Yang linked to, it begins by quoting two senses listed in the [日本国語大辞典]{にほんこくごだいじてん} which I will reproduce here in Japanese and translation:
④感謝したり詫びたりする気持ちを含む挨拶(あいさつ)に用いる。内容も省略し、「どうもどうも」と重ねて用いることも多い。(以下、用例略。)
⑤あいまいな、または安易な挨拶のことばとして用いる(以下、用例略。)
And my translation:
(4) Used as a greeting that include feelings of gratitude and apology. Frequently, both the contents [for which one is grateful or apologizing] are omitted and the word is repeated as "dōmo dōmo" (following usage examples omitted). (5) Used as a greeting ambiguously or lightly.