J.B.
My oldest son was dx'd with visual tracking problems in 2nd grade by his tutor and this was validated by some school testing at my request. My son also has some other learning disabilities and ADHD, so those may increase the effects of his visual tracking problems.
For my son, it was noticed when the tutor was doing some diagnostic testing. From what I recall, he was reading lists of words that were set up in three or four columns on a page. He was supposed to read left to right, row by row, and would either randomly read ahead or behind by a row or if the word he was supposed to read was "right" but the word above it was "lemon" he would pick up the L from the top word and say "light" instead of "right"." Another thing that he would do when reading pages of text was that his eye would focus on the rivers of white space between the words so he would lose his place easily. When writing, he would unconsciously indent each line on the page
kind of like this so that by the time
he got to the bottom of a page
there was very little room
left to write.
The other way this would manifest is that when reading, he would prefer to read in very low light and would hold the page at a weird angle because having a bright light over the page and reading directly over the page was hard for him. Finally, loathed dot-to-dot puzzles and word searches. Part of his accommodation plan later was that he was exempt from doing that kind of work - teachers would do these as a fun way to learn something and it drove him bananas instead.
He's in 8th grade now and it is something that he either grew out of or has gotten used to. He's not a great student or an avid reader, but he has no problem reading material that interests him. At one point, the learning specialist at school screened him for "Irlen Syndrome" which is a group of visual tracking symptoms and he did fit the criteria. He was offered a transparent overlay to use when reading and it definitely helped but he felt weird using it so he stopped using it after about a year.
I would guess that an OT working in a pre-school setting has the experience to accurately pick up on this but perhaps it is just a difference in setting that's distracting him. She should be able to tell you more about this as it relates to your son in particular, like what she is seeing that makes her think this, how consistent she thinks it is, and then if what you hear makes sense, find out more about what you can do at home to reinforce whatever she does in school. My son was much older when this was noticed (7 instead of 4) so it may be one of those things where, if caught early, some interventions can help strengthen this process.