The incident took place in the Pinebank area of Ladywell in Livingston, at around 6.50pm on Friday, 30 April, and was said to be disturbing for the woman who witnessed it.
Officers have issued a description of the suspect, and have urged the public to get in touch if they have information that can help.
The man is described as being white, in his 30s, around 5ft 8 with a very slim build.
He was wearing dark denim jeans, a dark khaki green Barbour style jacket with a hood, and was wearing round-framed glasses.
At the time, he was also understood to be wearing a blue surgical facemask.
Detective Constable Adam Gow, from Livingston CID, said: “Clearly, this was a disturbing and upsetting incident for the woman involved, who thankfully reported the matter to police.
“We are asking the public to get in touch if they think they know who this man is, or have witnessed similar incidents recently.
"We will be carrying out extensive enquiries to find the man responsible.
“If anyone has any information that could help with our enquiries, then please contact Police Scotland on 101, quoting incident 3046 of 30 April.
"Alternatively, make a call to Crimestoppers on 0800 555 111, if you wish to remain anonymous.”