Law enforcement sources tell TMZ ... the swine skull was discovered outside the LAPD's infamous Rampart station Saturday afternoon, and though the message is hardly subtle ... it's unclear if the gesture was intended as a serious threat.

We're told the department's looking into it further and reviewing surveillance footage in the area. Cops are also being advised to maintain a low profile right now while off-duty ... in order to avoid confrontations with a public that's heated about police practices. Cops are well aware that's why hundreds of thousands, in L.A. alone, have been marching.

As you know ... there have been several incidents of violence involving cops the past 2 weeks during the unrest over George Floyd's death, systemic racism and police brutality in the U.S.

Some of the more alarming incidents include a Chicago cop charging and punching a guy, cops in Buffalo pushing an elderly man to the ground and a vehicle running over an NYPD officer.

It's worth noting ... LAPD's Rampart Division made national headlines for a massive police corruption scandal in the late '90s with officers implicated in police brutality, drug deals and even bank robberies.