Thank you for your question and picture. It appears that your basswood tree may be suffering from a Basswood Leafminer. You mentioned that you haven't seen any Japanese beetles, which cause similar holes between the leaf veins. Japanese beetle damage is usually more cosmetic than fatal to trees and plants. Here is a resource from Extension about basswood/linden diseases:

What's wrong with my plant? : Garden : University of Minnesota Extension (umn.edu)

Here is another article about Basswood Leafminer from Ohio State University:

Basswood Leafminer Damage Becoming Evident | BYGL (osu.edu)

You will want to observe the entire tree and notice if you see any leaf dieback or discoloration of your tree. If your basswood tree is highly infested, I would suggest contacting a certified arborist in your area. Here is a link to find one:

How to hire a tree care professional | UMN Extension