Officers were called at 8.45am today (January 8) after the pool of blood was found on Uxbridge Road.
The major road was closed by a cordon and large police presence but no victim could be found.
A cordon remains in place as of midday and members of the public are being asked to contact police with any information.
A Metropolitan Police spokesperson: “On Thursday, 8 January at 08:45hrs police were called after blood was found on a pavement on Uxbridge Road, Stanmore.
“Officers attended the scene and enquiries are ongoing. No victim was identified during a search of the area.
“A cordon remains in place and anyone with information that may assist police is asked to call 101 quoting CAD 1498/08Jan.”
