dir T162511*.dwg /s /b >> searchresults.txt 2>nul || (>>searchresults.txt echo T162511.dwg not found)
If dir command does not find the indicated file, it raises errorlevel and the code after ||
is executed
EDITED - Code adapted to comments. Assuming it will be included in a batch file, and the drive:\path\to\file
is inside a variable (maybe defined from some parameters to batch file), this should to the work
set "filesToFind=c:\somewhere\T162511*.dwg"
dir /s /b /a-d "%filesToFind%" >> searchresults.txt 2>nul || (>>searchresults.txt echo %filesToFind% not found)