Mercruiser manufactured more than just a tks model 4.3 liter engine in 2007. They also produced efi models during that period. It's possible they purchased wire harnesses from another company and modified them to fit their engines. Or they made the harnesses and used a template/base type of harness that included wiring for multiple models of 4.3 engines.
My guess is that the wire(s) in question are for a different model engine than a tks but were added in the initial manufacture of the harness. If the harness was used on a tks engine no further modification was needed and the superfluous wires were tucked into the loom. Perhaps someone was digging in the loom to troubleshoot an electrical issue at one point and pulled the wire(s) out and never put them back
If none of the wires are live (12 + volts) or are merely grounds there's no need to worry
