Since yesterday's close, the share price has risen 7.5p (0.96%) to 792p after it traded 36p, or 4.5%, lower at 769p, one of the sharpest decliners in a weak FTSE 100.
In last year's acquisition of Y&R, staff agreed to retain two thirds of their shares for at least a year, effectively locking up around 20m WPP shares. A third of their holdings could have been sold in the last year, but the expiry date for the remaining two thirds came a few days ago.
A spokesman for WPP said the shares were only a small proportion of the company's total of 1.1bn shares. He added that there was no indication that Y&R staff wanted to sell.
A senior trader said stock was expected to be offered in the market, but so far none had.