Can Subversion record a merge without mergeinfo?

I'm searching through a particularly gnarly set of older revisions in one of our repositories, trying to figure out what happened with a couple of supposed merges.

When I look at one of the revisions in the target branch, it shows me content that's exactly the same as a revision on the source branch (trust me, I checked the diffs, too):

$ svn --log --verbose --use-merge-history --revision 100 ^/target
------------------------------------------------------------------------
r100 | <author> | <date> | 1 line
Changed paths:
   A /path/to/new/file
------------------------------------------------------------------------

But, there's something missing. There's no mergeinfo property change recorded anywhere, right? So it can't be a merge, as best I understand it. Maybe the author hand edited the file?

I double checked what Subversion thought were eligible revisions:

$ svn mergeinfo --show-revs eligible ^/source ^/target | grep 100

Nothing! Subversion thinks the revision has been merged despite the lack of mergeinfo.

$ svn mergeinfo --show-revs merged ^/source ^/target | grep 100
r100

Is that possible? How?

I read the CollabNet article and the Svn Book section on missing mergeinfo.

Merging unrelated sources: This is not the case, as I said, I can see the exact content/diff in the source branch.
Merging from foreign repositories: Ditto.
Using --ignore-ancestry: This is possible (I don't know what command the author invoked), but wouldn't this revision appear in the merge-eligible list?
Applying reverse merges from a target's natural history: It's obvious from history that this is not a reverse merge.

Solution

Oops, there actually is svn:mergeinfo property as far as I can tell now. I don't remember whether svn log prints property-only changes (it has to), try running svn log --verbose or run svn propget svn:mergeinfo --verbose against the ^/target.

Read SVNBook | Merges Without Mergeinfo.

BTW, don't forget that mergeinfo was introduced in Subversion 1.5. So if the merge happened with older Subversion version, there is a chance that this is why svn:mergeinfo property is missing.

And yes, the commit author out of ignorance could hand edit the file to make the commit look like a merge.