One possible explanation is that they are testing thousands of forums for unmoderated posting. By later searching for the unique string, they can readily identify which forums are vulnerable to more targeted link spamming and the like.

I actually had a post on a message board just last week that read "testing site security" followed by a spam URL. Fortunately, any message board I add to a site immediately sends a notice to the site owner as soon as a message is added and records the submitter's IP. Needless to say, the message was deleted and the IP blocked. But 99% of the time, it is a spam bot.

Marshall

...sends a notice to the site owner as soon as a message is added...

We've got a niche site that uses a blog because a forum isn't justified. However, we allow comments, and specifically allow anonymous comments if desired. No requirement of name, email, anything. They have these options, but if they simply want to add useful follow-up without signing in or providing 'required' information - no problem.

We have the advantage that the niche is small, the and we are a pretty small target for abuse. If big growth, then we might have to tighten up. Right now, the priority is user-friendly and minimum 'requirements'.

Only takes a moment to check out the comment addition. Only takes another moment to dump it, though not near the problem for us as for others.

I haven't seen meaningless posts but I have seen posts that were nearly meaningless. General or similar contents to the topic that could easily fit anywhere into a conversation.

I haven't actually seen it happen but I suspect some of them may be bots planting posts for editing in the future. Many forums offer indefinite editing of past posts. Many of these posts will appear in the off topic type forums which generally don't get the moderation normal forums would.

1. Human used to get by captcha.
2. Human picks forum ID's to post in that are unlikely to be moderated.
3. Bot loads forum, and a topic every few days. Parses HTML to get text and uses something like A.L.I.C.E. to generate somewhat intelligible response.
4. The posts may seem a little strange but since they contain no spam links they are overlooked by both moderators and members.
5. 1 year goes by and post is now gone into oblivion, bot comes back and edits the post adding spam content and links.

Seems like a lot of work but a lot of these spammers will find any popular board where links would be desirable impossible to get a post in where their spam links will do the most good.

I'm sure I'm not the first to think of it, never seen it mentioned though.

...sends a notice to the site owner as soon as a message is added...

Using Movable Type. Did not know this, but even with wide open permissions to most unmoderated anonymous posts it will flag suspicious posts as potential spam. In this case it will NOT post the post until moderated, i.e., a spam filter mechanism to require that the post be moderated and allowed. Didn't even know it did this until recently. Got a couple of blog comments that should have posted but were flagged. They were both completely bogus, so I just clicked the provided 'report as spam' button. Pretty slick. Overlinked and without content seemed to be the filter trigger. So, I didn't even have to delete - simply denied the posts.