Check with your email provider what the retention period is on their server.

@christ1 - Thanks for the reply. I run my own IMAP server. As I mentioned in my original post I can see in Thunderbird's activity log that thunderbird is doing the deletion -- "Deleted 507 messages from Trash"

I wish there was a way to access/export the Thunderbird activity log as a text or something searchable.

Check this article for other possible reasons. http://kb.mozillazine.org/Disappearing_mail

Are there any other devices accessing the same server. They may be the reason for deleting messages.

Note, with IMAP, all messages are managed on the server. When you see messages being deleted in Thunderbird, this possibly just shows Thunderbird catching up and synchronizing the changes already made on the server.

@christ1 - So if I understand what you are saying when thunderbird's activity log shows "Deleted 507 messages from Trash" it could either be because Thunderbird actually deleted the messages in the IMAP store, or because Thunderbird has detected that those messages were deleted from the IMAP store and is now reflecting the delete in it's local cache?

Exactly right. The latter sounds more likely to me though.