Totally agree here - would be easy to fabricate DNA oligos with given mtDNA or STR sequences, especially if you know forensic investigators are only going to use very small target sequences for identification purposes.

This could be a case for using shotgun / whole genome sequencing - we'd expect to see a more even distribution of genome coverage than one would get by leaving targeted oligos behind. But this isn't likely to happen anytime soon; the costs are far too high (10-100X greater than targeted sequencing).

In the meantime though, one highly feasible avenue for spotting synthetic DNA oligo fragments is the presence and position of nucleosomes (https://en.wikipedia.org/wiki/Nucleosome), which are DNA-associated protein complexes that occur in DNA from organisms, but not in synthetic DNA oligos. These have been used successfully to trace tissue of origin in cell free DNA in humans, and have specific signals related to chromatin/genome topological state. The only way I can think of to fake this signal would be to have a cell culture from the person you're trying to imitate. Granted, this too is not completely unreasonable - there are now very robust commercially available protocols for deriving iPSCs from small dermal fibroblast samples.