There are two email sound settings, one for sent and one for received. Normally they are set to Swoosh for sent and (I don't remember because I turned it off) for received.
If the sound you heard was set for sent, and only sent, then it probably means an email you tried to send earlier was sitting in the Outbox waiting for a good connection. Or if it was an email with a large attachment, the iPad finally finished sending it.
Hold in mind you can initiate emails not only from the Mail app, but from other apps as well. For instance, if you sent a photo (or photos) to someone from the Photos app it may have taken a while to finish sending. A slow internet connection will do this. Also, if you spent most of the day off line, or on a 3G connection, the emails may have been delayed getting sent.
If you are curious:
Open your Mail and view it in Landscape. Tap the Mailbox button above the email list. Keep tapping it (if necessary) until you see two lists, one with your Inboxes and the other with your accounts. Open the account and look in the Sent folder. The email that just got sent should be on top (or near the top if you've sent more since).