Its a number one highlited in yellow of course
Joking aside, is this data in the field or is it overlaying the field, i.e as if it has been added in layout mode?
In the formatting bar there is a highlite option which adds a yellow background to content, could this be it?
Failing that, a little more info may help:
Is it an editable field or a calculated field?
Does the field contain conditional formatting?
Theres quite a lot it could be in my opinion, but If layout mode is accessible, it sounds to me like they have pressed CTRL+L then typed 1, then found their way back into Browse mode not thinking anything has changed.
Typing anything in layout mode without an object selected will put a text box on the layout, if auto save layouts is set this could go un-noticed at first.
Viola! It is a tiny text field with a number one in it, hilighted. My
volunteer doing data input inadvertently hit a CTRL+L and then somehow
created a field and put a highlighted 1 in it. It's curious because he
knows nothing about editing layouts so it was just random clicking and
typing. Thanks for the help.
Really, basic privileges should be established. Simple data entry even, for everyone but the developer can offer great protection. Otherwise more damage (much worse) can be done by accident. :-)
Especially since you state "he knows nothing about editing layouts".
In fact, even if the users did know, it still does no harm to add a layer of protection to prevent accidental changes such as this.
As LaRetta quite rightly says, much worse can be done by accident.