W.P.
Dear D.:
Just in case it comes up again (or some other time), it depends on the type of sound and how long she overheard it. If she only listened in briefly, a moan could be 'explained' with a 'having a cramp' or 'receiving a massage', and higher pitched sounds with 'daddy tickled me' (which may not be too far from the truth). I agree that being 100% truthful may make it harder on your daughter. If she listened in longer or you used 'bad' language, you may just have to say that it was something between adults. Either way, her main concern may be that 'daddy hurt you' for the lack of better understanding. Of course, this would be aggrevated if you practised anything rougher or out of the ordinary (e.g., BDSM). No matter what your explanation, make sure to add that you are OK and that you did not get hurt. She may not ask for it directly, but I would say it anyway to put her mind at ease.
In case you ever show marks like a 'hickey' or 'rugburn', think of a good excuse as soon as you notice. A hickey could be a mosquitoe bite, and for rugburn you may say that you had to get something from under the bed.
When she gets older, you may want to establish that you and dad have alone time. Nonetheless, if she knocks on the locked door, you should answer it soon to show her you are OK.
Tickling and massage are usually good excuses, because it explains why dad was right there. After all, he WAS the reason for your sounds.
Regards,
W.