It is estimated that 40 to 50% of the infected patients are a or pauci-symptomatic.
The screening test (PCR) detects the virus when located in the nasopharyngeal are (where the sample is taken).
If you have severe symptoms (cough, fever, headache, anosmia…), there are logically more chances of detecting the virus than if you carry no symptoms.
Global efficiency of the PCR test is currently estimated at 70%, which means the in 30% of the cases, your result is negative, but you are still a carrier of the virus and hence – potentially contagious, even if the chances of contamination are probably smaller than if you were positive with strong symptoms.
As a conclusion: if the test is negative, you still have a contamination risk. This risk also depends on whether Luxemburg-city is a very active Covid19 zone.
Best would be, of course, to quarantine yourself (15 days, ideally).