Do bailiffs always leave a letter?

No

Enforcement regulations provide for notices when any of the following happens:

The bailiff must give a Notice of Enforcement at least than seven clear days,^[1] before taking control of goods.^[2][3]

If the bailiff intends to enter premises using reasonable force,^[4] the debtor must be given notice,^[5] not less than two clear days before, excluding public holidays and Sundays.^[6]

After entering, re-entering the premises, the bailiff must give notice telling the debtor what the bailiff is doing.^[7][8][9]

If the bailiff has taken control, of goods, he must give an inventory, which is a list of goods he had taken.^[10]

If the bailiff has clamped a car outside, or on a highway, the bailiff must give additional notice.^[11][12]

If the bailiff has removed goods for sale or storage, he must give an additional notice saying where the goods have been taken, and what steps the debtor can take to reclaim them.^[13]

If the bailiff is selling the goods, the bailiff must give a Notice of Sale,^[14] at least seven clear days before selling them,^[15] which must state the place, date and time of the intended sale.^[16][17]

If the bailiff breaches any of these rules, the debtor can sue.^[18]

Bailiffs often leave red Final Notice or a locksmith threat, when calling and nobody was home. These are not a statutory notices. It may even be a bailiffs phishing letter.^[19]

^{[1] See Giving a Notice of Enforcement
[2] Paragraph 7.1 of Schedule 12 of the Tribunals, Courts and Enforcement Act 2007
[3] Regulation 6(1) of the Taking Control of Goods Regulations 2013
[4] See Bailiffs and threatening to enter with a locksmith
[5] Paragraph 19A of Schedule 12 of the Tribunals, Courts and Enforcement Act 2007
[6] Regulation 25 of the Taking Control of Goods Regulations 2013
[7] Regulation 26 of the Taking Control of Goods Regulations 2013
[8] Paragraph 28 of Schedule 12 of the Tribunals, Courts and Enforcement Act 2007
[9] Regulation 30 of the Taking Control of Goods Regulations 2013
[10] Paragraph 34 of Schedule 12 of the Tribunals, Courts and Enforcement Act 2007
[11] Paragraph 33 of Schedule 12 of the Tribunals, Courts and Enforcement Act 2007
[12] Regulation 31 of the Taking Control of Goods Regulations 2013
[13] Regulation 32 of the Taking Control of Goods Regulations 2013
[14] Regulation 38 of the Taking Control of Goods Regulations 2013
[15] Paragraph 40(1) of Schedule 12 of the Tribunals, Courts and Enforcement Act 2007
[16] Regulation 39 of the Taking Control of Goods Regulations 2013
[17] See Bailiff sold your car without giving a notice
[18] Paragraph 66 of Schedule 12 of the Tribunals, Courts and Enforcement Act 2007
[19] See Bailiffs leaving a phishing letter or document}