Emission-line stars are typically surrounded by dense circumstellar material, often in form of rings or disc-like structures. Line emission from forbidden transitions trace a diversity of density and temperature regimes. Of particular interest are the forbidden lines of O i λλ6300, 6364 and Ca ii λλ7291, 7324. They arise in complementary, high-density environments, such as the inner-disc regions around Be supergiants. To study physical conditions traced by these lines and to investigate how common they are, we initiated a survey of emission-line stars. Here, we focus on a sample of nine Be stars in different evolutionary phases. Emission of the O i lines is one of the characteristics of Be stars. We find that four of the objects display Ca ii line emission: for the Be supergiants V1478 Cyg and 3 Pup, the kinematics obtained from the O i and Ca ii line profiles agrees with a Keplerian rotating disc scenario; the forbidden lines of the compact planetary nebula OY Gem display no kinematical broadening beyond spectral resolution; the luminous blue variable candidate V1429 Aql shows no O i lines, but the profile of its Ca ii lines suggests that the emission originates in its hot, ionized circumbinary disc. As none of the Be stars of lower mass displays Ca ii line emission, we conclude that these lines are more likely observable in massive stars with dense discs, supporting and strengthening the suggestion that their appearance requires high-density environments.